Collective energy absorption of ultracold plasmas through electronic edge-modes

Abstract

Abstract. We investigate the collective dynamics of electrons in ultracold neutral plasmas driven by an oscillating radio-frequency field. We point out the importance of a sharp density drop at the plasma boundary that arises due to unavoidable charge imbalances, and show that this plasma edge provides the major mechanism for energy absorption from the external field. Using a cold fluid theory, we derive the corresponding absorption frequency and validate our findings by microscopic molecular dynamics simulations. The proposed edgemode is shown to provide a consistent explanation for the observed absorption spectra measured in different experiments. Understanding the response of the electronic plasma component to weak external driving is essential since it grants experimental access to the time-evolving density and temperature of ultracold plasmas. Contents

Heating mechanisms in radio-frequency-driven ultracold plasmas

“…The resulting absorption rate, divided by 2U p for comparison, gives again the hard-wall collision frequency Eq. (14), with v the highfrequency velocity. In an UCP, however, the Brunel mechanism is not in effect either, since typically the skin depth, which is comparable to c/ω p with ω p the plasma frequency, is much larger than the plasma size.…”

“…Note that this behavior is not at all described by the hard-wall approximation Eq. (14). The reason for this is that an electron bouncing between hard plasma boundaries abruptly changes its velocity at every wall collision, giving rise to high-frequency components essentially regardless of the velocity.…”

“…This is particularly relevant under conditions of resonance with plasma modes, in which case the absorption of rf energy by the UCP is dominated by the strong dependence of E 0 on the driving frequency [13]. The determination of the frequency response of the UCP, and hence the polarization fields, is actively being studied [11][12][13][14], but is outside the scope of this paper. Nevertheless, our results may be directly applied once E 0 is known.…”

Ion temperature evolution in an ultracold neutral plasma

“…This study adds to prior studies of fundamental plasma properties conducted using UNPs, such as the plasma creation process, 3,4,13,14 collective modes of ions 16,17 and electrons, 13,18,19 and formation and ionization of Rydberg atoms in the plasma.…”

Ion holes in the hydrodynamic regime in ultracold neutral plasmas

“…This makes them an excellent platform for studying a wide range of plasma phenomena, such as equilibration of strongly coupled plasmas, [3][4][5][6][7][8][9][10][11][12][13][14] ambipolar diffusion with 15 and without 16,17 a magnetic field, electron plasma oscillations, 16,18 Tonks-Dattner resonances 19 and edge modes, 20 ion acoustic waves, 21 an electron drift instability, 22 threebody recombination at ultracold temperatures, 13,[23][24][25][26][27][28][29] and the crossover to an ultracold plasma from a dense gas of Rydberg atoms. [30][31][32][33] Recent experiments creating ultracold plasmas in a seeded supersonic molecular beam 34 introduce molecular processes to the plasma evolution and show promise for yielding more strongly coupled systems.…”